It is well known to feed yarn to machines making use thereof, particularly the weft yarn to looms, by means of feeding devices or yarn feeders. Such devices are positioned between the spool and the loom and are meant to temporarily store the weft yarn. They also facilitate its unwinding from the spool, and subsequently feed it to the loom insertion members with optimal and preset yarn tension values.
The type of structure universally adopted at present for such feeding devices involves winding the yarn into successive turns around a winding unit or drum, kept motionless, by means of a winding arm moved by an electric motor. Means for detecting the amount or reserve of yarn present on the winding drum are provided to control the running and rotation speed of the motor of the feeding device according to the amount of yarn drawn by the loom; so as to make the yarn unwinding speed from the spool as uniform as possible.
When yarn feeders are used on gripper or projectile looms, they comprise at their outlet end yarn braking means, positioned downstream of the yarn winding drum and meant to feed the weft yarn to the loom with a specific desired tension. Many of the known yarn braking devices act directly on the outlet end of the drum and are positioned upstream of an outlet yarn guide of the feeder, meant to ensure a correct unwinding of the weft yarn.
Most yarn feeders make use of brake devices consisting of a plurality of natural or synthetic bristles, fixed to a support in the form of a closed ring carried by a bracket movable along the main axis of the yarn feeder. The plurality of bristles bear--with a variable pressure adapted to be preset by adjusting the axial position of the bracket--onto the outer periphery of the yarn reserve winding drum of the feeder.
Other yarn braking devices have been conceived, as an alternative to the ones with bristles: and their use has proved advantageous to overcome the drawback of the rapid wear of the bristles. The braking element in these devices comprises a plurality of thin sheet-metal strips, positioned side-by-side onto a frustoconical surface and pressing onto the outlet end of the feeder drum. To guarantee--by a self-adjustment of the strips--an efficient and continuous-contact of the strips with the winding drum, the said braking element is preferably mounted onto a cup-shaped support with open bottom, carried by a bracket of the yarn feeder the position of which is adjustable along the drum axis. A yarn braking device with sheet-metal strips of this type, which has proved satisfactory and has found a widespread use, is disclosed in EP-B1-49897 filed by the same Applicant. In this brake device, the sheet-metal strips are in the form of thin blades and the cup-shaped support is mounted on the movable bracket of the yarn feeder by way of a double articulation, so as to be able to perform limited oscillations about two axes perpendicular to each other and to the axis of the feeder drum.
More recently, brake devices for yarn feeders having still different characteristics have been conceived. These devices comprise a continuous braking band carried by a plurality of tongues with varying flexibility. The tongues are self-adjustable in that they are mounted movable into a fixed support. Examples of such brake devices are described in PCT/EP93/03086 and PCT/EP93/03262, both filed by the same Applicant. Due to the particular configuration of these devices and, above all, to their mounting--which allows the braking element to be widely deformed and to perform short movements in every direction in the support (thus acting like a universal joint, adapted to guarantee all degrees of freedom)--a perfect adjustment of the braking element in respect of the drum is obtained in the tangent area of contact, particularly taking into account the deformations caused by the passage of the weft yarn, and this allows to guarantee a very regular yarn tension diagram.
Even further improvements in the behavior of the brake device, in its area of contact with the feeder drum--allowing to simultaneously obtain a yarn tension trend, during weft insertion, with no harmful peaks (so as to ensure a good result in the weft insertion process)--have been accomplished with the yarn braking device proposed by the Applicant in PCT/EP94/00476. In this case the device, centered on the drum axis and adjustable along the same, comprises a frustoconical braking element with continuous surface and varying flexibility, carried by a stiff ring supported to which it is fixed close to its major circumference. The support is in turn fixedly mounted onto a bracket of the yarn feeder, the position of which is adjustable along the drum axis. Similar brake devices--wherein the braking element consists of a continuous surface supported by a highly flexible member, such as to allow a self-adjustment of the continuous surface in respect of the outlet end of the winding drum--are described in EP-A1-534263 in the name of L.G.L. Electronics.
From a careful consideration of the techniques described heretofore and their developments, it can be noticed that brake devices for yarn feeders have evolved towards the use of a braking element with continuous surface, which seems to be more suited than others to solve the more and more complicated problems faced by the experts as a result of the advanced state of technique in this field and its constant profess.
It can be noticed moreover that the already known types of brake devices for yarn feeders: including the most evolved ones, while allowing to obtain a satisfactory braking effect for weft yarn insertion purposes, are not adapted to conveniently solve the problem of the proper adjustment of the braking element in respect of the outlet end of the winding drum, when--as it happens fairly often, for various reasons, in the industrial production process--in the brake devices which are introduced on the market (either new, or after a more or less long period of use): the braking element is not perfectly aligned with the winding unit of the yarn feeder.